The common mechanical angular rotational power transmissions are in two basic types; gear types, and universal joint types.
In gear type transmissions the rotational motion and torque are transmitted by mashing teeth of a pair of gears. The exact alignment of the gears is maintained by bearing supported shafts in a rigid housing. Any flexibility for axial displacement of the shafts is provided externally to the housing by telescoping, or splined shafts. The speed ratio of the input and output shafts is determined by the number of teeth of the gears. The rotational motion is transmitted at steady, constant angular velocity. The angle of intersection of the axes of rotation of the coupled shafts is constant.
Universal joint type couplers employ two forks attached to the coupled shafts, and a yoke which is journally attached to both forks. Universal joints require neither rigid housing nor bearing support for the shafts, as alignment of the coupled shafts is provided by the forks and the yoke. Flexibility for axial displacement of the coupled shafts is provided by telescoping, or splined shafts. The speed ratio of the transmitted rotational motion is one-to-one. The rotational motion is transmitted at a cyclic, varying rate during each revolution, which is directly proportional to the angle of intersection of the coupled shafts. The angle of intersection of the axes of rotation of the coupled shafts is variable between about 0 and 45 degrees.